Microcavity plasma devices produce a nonequilibrium, low temperature plasma within, and essentially confined to, a cavity having a characteristic dimension d below approximately 500 μm. This new class of plasma devices exhibits several properties that differ substantially from those of conventional, macroscopic plasma sources. Because of their small physical dimensions, microcavity plasmas normally operate at gas (or vapor) pressures considerably higher than those accessible to macroscopic devices. For example, microplasma devices with a cylindrical microcavity having a diameter of 200-300 μm (or less) are capable of operation at rare gas (as well as N2 and other gases tested to date) pressures up to and beyond one atmosphere.
Such high pressure operation is advantageous. An example advantage is that, at these higher pressures, plasma chemistry favors the formation of several families of electronically-excited molecules, including the rare gas dimers (Xe2, Kr2, Ar2, . . . ) and the rare gas-halides (such as XeCl, ArF, and Kr2F) that are known to be efficient emitters of ultraviolet (UV), vacuum ultraviolet (VUV), and visible radiation. This characteristic, in combination with the ability of microplasma devices to operate in a wide range of gases or vapors (and combinations thereof), offers emission wavelengths extending over a broad spectral range. Furthermore, operation of the plasma in the vicinity of atmospheric pressure minimizes the pressure differential across the packaging material when a microplasma device or array is sealed.
Research by the present inventors and colleagues at the University of Illinois has resulted in new microcavity plasma device structures as well as applications. A particularly promising class of microcavity plasma device arrays is formed in metal and metal oxide. Large-scale, low-cost arrays of devices that can be flexible are formed by inexpensive processes with metal electrodes encapsulated in metal oxide. One problem that has arisen with such devices is the occurrence of defects in the oxide layer that encapsulates and protects the metal electrodes from plasma generated in the microcavities. During the formation of oxide over metal in which microcavities are formed in a metal sheet, the oxide can develop defects such as cracks. This is especially true in areas where the oxide is formed over the edges of microcavities.
Important arrays of metal and oxide microcavity plasma devices have been provided by past work at the University of Illinois. For example, Eden et al., U.S. Pat. No. 7,573,202 discloses metal and metal oxide arrays that are formed by growing a nanoporous dielectric on a metal substrate in which microcavities have been formed. Subsequent anodization forms a nanoporous oxide, which can also be backfilled with dielectrics, metals or carbon nanotubes, for example. This provides a high performance dielectric. However, the nanoporous dielectric can develop cracks, especially in areas near the rim of a microcavity.
The formation of microscopic cracks in the dielectric of a microplasma device can limit device lifetime and cause operational flaws. The cracks provide a pathway for dielectric breakdown that can disable portions or all of an array of devices. The excellent electrical breakdown characteristics of nanoporous alumina (Al2O3), for example, are of little consequence once a crack of sufficient size appears in a thin layer of the material.
Past efforts have been made to mitigate the appearance of cracks. One solution is to apply thin glass films to regions where cracking occurs. This increases cost and complexity in the manufacturing process. Another drawback is that glass has a melting point well below that of dielectrics such as Al2O3, which can limit operation of devices and arrays formed this way. The glass also generally overcoats the nanostructured alumina, reducing the dielectric strength offered by the network of hexagonal pores from which the alumina film is composed.
Eden et al, U.S. Pat. No. 8,004,017, discloses large arrays of metal/metal oxide microplasma devices and a fabrication method for the same. High quality, large arrays are formed. The fabrication method is a wet chemical process in which self-patterned circumferential electrodes are automatically formed around microcavities during an anodization process that converts metal to metal oxide. The size and pitch of the microcavities in a metal foil (or film) prior to anodization, as well as the anodization parameters, determine which of the microcavity plasma devices in a one or two-dimensional array are connected. The metal foil is obtained or fabricated with microcavities having any of a broad range of cross-sections (circular, square, etc.). The foil is anodized to form a nanostructured metal oxide layer. One or more self-patterned metal electrodes are automatically formed and simultaneously encapsulated in the metal oxide created by the anodization process. The electrodes form uniformly around the perimeter of each microcavity, and can be electrically isolated or connected in patterns. The shape of the electrodes that form around the microcavities is dependent upon the shape of the microcavities prior to anodization. The metal oxide formed by this method can also develop microcracks, especially in areas which traverse (or span) the rim or other sharp discontinuity associated with a microcavity. Areas immediately adjacent to such discontinuities are also susceptible to microcracks. With densely packed and larger scale areas, cracking is more prevalent.
Limiting the number of defects in large and densely packed arrays of microcavities and microcavity plasma devices has been accomplished with stress-reduction structures and techniques. Eden et al. U.S. Patent Publication No. US 2010-0001629 provides such arrays via stress reduction structures, geometries and fabrication techniques that limit the tendency of large scale and densely packed arrays to crack and buckle due to mismatches in the coefficients of thermal expansion between the oxide and metal.
The present inventors have determined a significant cause of microcracking in metal oxide films, and methods to avoid such microcracking. Generally, nanoporous oxide grown from a flat or gently-curved surface will be high quality, presuming that all other growth parameters (such as temperature and rate) are chosen judiciously. In an oxide layer that is essentially free of microcracking, the axes of the nanopores in the metal oxide are approximately parallel and oriented orthogonal to the plane of the metal substrate surface from which the oxide is grown. However, growing metal oxide film(s) from a surface having a sharply-rounded edge (such as that at the rim of a hole) generally introduces cracking in the film because the axis of the pores must rotate through a large angle (typically 90°) over a short distance. This situation introduces considerable strain into the film and subsequent cracking. This reduces the dielectric quality of the film.